Re: [Xen-users] 'xl reboot domid' ignores all in-memory changes AND the domu's config file changes

["Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>]

On 2016-09-21 15:33, Simon Hobson wrote:
> Austin S. Hemmelgarn <ahferroin7@xxxxxxxxx> wrote:
>
> A great description of self balloning and how to set it up. Not something I 
> need, but very interesting to the geek in me ;-)
>
> > 2. If you're doing this to overcommit memory (like I am), you need to have a 
> > very good understanding of your workload's memory requirements and how 
> > everything behaves, ...
> I second this. it probably comes under the heading of "if you need to ask what the 
> risks are, you shouldn't be doing it". Having seen the results of over-commit in 
> disk storage and the hassles it causes, I don't think it's possible to stress too highly 
> how big an issue it is when things ask for the resources you've promised them but don't 
> have.
> Fortunately, in the cases I've seen, it's been Windows HyperV and the 
> hypervisor pauses all the VMs to prevent the train wreck that would otherwise 
> happen. Still a PITA as the admin (not me !) has to find enough disk space to 
> be able to unpause something and free up space within the virtual machine's 
> disk - something of a catch 22 !
Yeah, although with memory there's better options to make sure you don't 
hit issues.  In my case, I've got about 6 VM's on my home server using 
this, each providing a specific network service.  Within each of them, 
everything that runs for more than a few seconds is confined in a Linux 
control group set to cap max memory usage.  Because of this, any runaway 
programs will get caught with a localized OOM condition (within the 
cgroup instead of the system) before they can allocate enough to cause 
other issues.  As a result of this, the only times the system might be 
vulnerable to memory exhaustion are when the VM's are booting (which 
I've got covered by serializing VM startup), and when I'm running 
updates on the VM's (which I also serialize).
> > Pretty much everything I know about it is stuff I've figured out from reading 
> > source code.  What I've got above is as close to a complete description as I've 
> > ever seen, maybe it should go on the Wiki?
> I think it would be a very valuable wiki page.
I would think it should go on the transcendent memory page (which is in 
severe need of an update), or be linked from it, seeing as it's pretty 
much only possible because of transcendent memory support in Xen.  I 
would put it up myself, but I don't have an account, and I really don't 
have the time to deal with getting one.  It may also be useful to list 
what Linux distributions have kernels built with the required options. 
I can't help much with this though because I run custom kernel builds on 
all my systems.
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